In automation technology, especially process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are sensors, such as, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity, respectively. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, or the fill level in a container, can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. Besides such sensors and actuators, also referred to as field devices are generally also such units, which are participants in a fieldbus and which are able to communicate with superordinated units, so that e.g. remote I/Os, gateways, linking devices and radio adapters are also field devices. A large number of such field devices are available from the Endress + Hauser group of companies.
In modern industrial plants, field devices are, as a rule, connected with superordinated units via bus systems, thus bus systems such as Profibus®, FOUNDATION Fieldbus®, HART®, etc. Normally, the superordinated units are control systems, or control units, such as, for example, a PLC (programmable logic controller). The superordinated units serve, among other things, for process control, process visualizing, process monitoring, as well as for start-up of field devices. The measured values registered by the field devices, here especially field devices in the form of sensors, are transmitted via the particular bus system to one or, in given cases, a number of superordinated unit/units. Along with that, there occurs also, for the purpose of configuring, parametering, diagnosis of field devices or for the purpose of operating actuators, data transmission from the superordinated unit via the bus system to the field devices.
Besides data transmission by wire between the field devices and the superordinated unit, wireless data transmission, or radio transmission of data, is becoming ever more important. Especially in the bus systems, Profibus®, FOUNDATION Fieldbus® and HART®, wireless data transmission via radio is the subject of specifications. Furthermore, radio, or wireless, networks for sensors are specified in greater detail in the standard IEEE 802.15.4. For implementing wireless data transmission, newer field devices, especially sensors and actuators, are partially embodied as radio field devices. These contain, as a rule, a radio unit and an electrical current source as integral components, wherein the electrical current source implements an autarkic electrical current supply of the field device.
Furthermore, there is the opportunity that field devices without radio units, especially sensors and actuators, be upgraded to become radio field devices by connection of wireless adapters that have radio units. A wireless adapter is thus a unit, by which a “conventional” field device, which is designed only for wired connection to a fieldbus, is upgraded to become a radio field device. Such a wireless adapter is described, for example, in the publication WO 2005/103851 A1. The wireless adapter is connected, in such case, to a communication interface, especially to a fieldbus-communication interface, of the field device, wherein the connection between wireless adapter and field device is, as a rule, releasable. Via the fieldbus communication interface, the field device can transmit the data to be transferred via the bus system to the wireless adapter, which transmits such via radio to the target location. Conversely, the wireless adapter, or the radio adapter, can receive data via radio from the gateway and/or the superordinated control unit and forward such via the fieldbus communication interface to the field device. The supplying of the field device with electrical power occurs, as a rule, via an electrical current source, which is associated with the radio adapter or with the field device. The electrical current source is usually a single-use battery or a rechargeable battery.
Radio, or wireless, networks in automation technology are distinguished by small energy consumption and use of a narrow transmission band. The narrow transmission band assures reliable data transmission to an addressed participant/field device at all times. A great advantage in the case of radio, or wireless, networks compared with wired bus systems is the saving of wiring and the avoiding of the therewith associated maintenance. Usually, field devices integrated in a radio network are—as already mentioned—fed via a single-use battery, thus an energy supply unit with limited power capacity. The service life of the single-use battery should, as a rule, be between 5 and 10 years.
Radio, or wireless, networks for automation technology, such as, for example, wireless HART or ISA 100, are embodied as mesh networks of various forms. Attention is paid that each field device, or, generally stated, each node, can communicate with at least two additional nodes, whereby the necessary redundance is created.
An autarkic node for a mesh network is described in WO 2005/094312 A2. In radio, or wireless, networks, communication usually takes place only at certain times. Outside of the active operating phases, the field devices are placed in a resting phase for the purpose of saving energy. In the resting phases, energy consumption is, at least approximately, zero. As already earlier mentioned, it is usual in the case of mesh networks that two nodes, or two field devices, communicate with one another in a narrow frequency band during the relatively short time of a so-called time slot. Furthermore, the center frequency of the frequency band changes according to a predetermined pattern—this method is usually referenced with the technical term “frequency hopping”. Frequency hopping reduces the disturbance susceptibility of a radio network.
The integration of a new node, e.g of a replacement—or of an additional field device, into an existing radio network is relatively time intensive, since, for the purpose of integration of an additional field device, a free communication channel, which is defined by the integration parameters, time window and frequency, must be provided. Modern mesh networks are designed to be self-organizing. For this, each node has a synchronization protocol, which enables it to detect neighboring nodes, to determine strength of the transmitted signals, to obtain synchronization information and information concerning frequency hopping and to ascertain the direct radio connections to the neighbors. Apart from the fact that the synchronization protocol must be provided to each field device, there is the problem that the field device during the integration phase has a high energy consumption, which—as already mentioned—in the case of battery operated field devices with a limited power capacity, leads to a shortened service life.
In order to service the field device, in the case of the known solutions, the field device must already be a participant in the radio network; alternatively, a wire connection to the field device must be produced.